Temperatures reached 32.1°C in the city at around 5.30pm on Thursday - equalling the hottest day of the year in Yorkshire.
The warm weather is expected to continue into Friday, with temperatures overnight not expected to drop below 20°C.
But occasional thunderstorms are possible before clearing on Friday morning to leave a hot, humid day until the evening when there is once again the chance of thunderstorms.
The maximum temperature for Sheffield on Friday is 34 °C.